A typical example of the mixture is municipal waste. Such municipal waste is generally composed of different kinds of waste such as garbage (food waste), wastepaper, wood chips, wastefiber (cloth), plastics, glass, metal chips or the like and these constituents are randomly mixed in the municipal waste which is collected. The respective constituents are of different physicial properties and, therefore, it is relatively difficult to sort and recover specific kinds for re-utilization. Further, it is also generally required to apply some pre-treatment processes to the municipal waste so as to perform sorting efficiently. The pre-treatment may be pulverization by a pulverizer to reduce and segregate the different constituents to a single constituent, respectively and to make the size of the pulverized constituents substantially uniform in size. However, it is difficult for the pulverizer of the prior art to uniformly and effectively pulverize the municipal waste comprising several constituents. Further, the prior art pulverizers consume much power and their elements are subjected to relatively rapid wear. Thus, a process utilizing this prior art pulverizer requires high operating and maintenance costs and is not satisfactory from an economic view point.
Thus, a method and apparatus have been developed to obviate the drawbacks above by paying particular attention to the difference in physical properties between the constituents of the object mixture to be treated. That is, the development was aimed at positively utilizing such characteristic differences and this resulted in a different pulverization time for each of the constituents to be pulverized to obtain particles or pieces of a certain uniform size. Consideration of such time difference leads to construction of a drum-type pulverizer and to feeding the constituents axially therethrough so that the pulverized constituents could be discharged at different axial positions depending on the progress of pulverization which, to a certain degree, is a function of elapsed time.
Thus, it is recognized from the above, that controlling the residence or staying time and axial advancement of the object mixture in a drum type apparatus is one of the important factors in this invention.
Also, some parts of the prior art pulverizers, such as beaters, are often damaged by relatively stiff constituents contained in the mixture which might result in shut-down of the operation and/or damage to the apparatus. Therefore it has been also desired to provide an apparatus which is free from such disadvantages.
Further, the prior art apparatus is relatively large and long. As a result, in the main shaft, which normally carries a plurality of elements such as beaters and/or anti-entangling plates, sometimes deflects due to its weight. If such deflection occurs, it may be difficult to maintain proper gaps between the tips of the beaters and the inner surface of the drum or the elements mounted on the inner side of the drum. One way to overcome the problem of deflection is to make the shaft more rigid, but this results in an increase of weight. It is preferable to increase the stiffness of the shaft without a remarkable increase in the weight and without reducing efficiency.